Several different lines of evidence point to a potentially important but not well-understood interaction between Alzheimer's disease (AD) and cholesterol dynamics. Apolipoprotein E4, a protein that binds and transports cholesterol and other lipids, has been identified as a risk factor for familial and sporadic AD. Brain cholesterol content was altered in autopsy samples of AD. Amyloid beta-peptide (Abeta), the main component of neuritic plaques seen in brains of AD patients has a multi- faceted interaction with cholesterol. Our overall hypothesis is that brain cholesterol dynamics are disrupted by AB. We propose that cholesterol transport into and out of cells and cellular distribution of cholesterol are altered by AB. Experiments of this application will examine potential mechanisms of effects of AB on aspects of those cholesterol dynamics. The disruption in cholesterol dynamics primarily involves the physico- chemical interaction of AB with lipids and proteins and AB-induced oxidation. Data in support of this application from our laboratory show that: 1) both soluble and aggregated AB increase fluidity of synaptic plasma membrane and this effect was inhibit by the anti-oxidant, Troxlox; 2) soluble AB partitions into the hydrophobic environment of SPM and aggregated AB is positioned near the membrane surface where it could interact with lipoproteins and their receptors; 3) aggregated AB binds cholesterol with a markedly higher affinity than saturated fatty acids, and phosphatidylcholine (PC); 4) AB increases the influx of cholesterol complexed with apoE4 and PC into cells; 5) AB inhibited HDL-mediated reverse cholesterol transport but facilitated removal of cholesterol in the absence of HDL; and 6) soluble AB increased cholesterol and aggregated AB reduced cholesterol in Golgi of neurons and astrocytes. Mechanisms that explain effects of AB on cholesterol transport and cellular distribution are not well-understood. Our hypothesis is that AB modifies cholesterol transport and distribution are not well-understood. Our hypothesis is that AB modifies cholesterol transport and distribution by the following proposed mechanisms: 1) AB increases the transport by altering the structure of apolipoprotein domains and lipid domains of lipoproteins, and inducing oxidation; 2) Soluble and aggregated AB differentially affect lipoprotein-mediated cholesterol influx by their actions on lipoprotein structure; 3) Differential action of AB on cholesterol influx mediated by the low density lipoprotein receptor-related protein (LRP) and the low density lipoprotein receptor (LDLR); 4) AB alters reverse cholesterol transport by modification of cellular distribution of cholesterol, disruption of the Golgi apparatus, cholesterol esterification, oxidation, and direct interaction with apolipoproteins and lipid domains. Cholesterol is essential for optimal cell function and instability in cholesterol dynamics could certainly contribute to the pathophysiology associated with AD.